This invention relates to a helicoid structure used to drive a lens barrel together with a photographing lens held therein along an optical axis in a zoom lens apparatus, vari-focal apparatus or the like adapted to vary a focal distance of the photographing lens.
A zoom lens apparatus or a vari-focal apparatus usually uses a mechanism for moving the lens barrel together with a photographing lens held therein back and forth along an optical axis and thereby to vary the focal distance of the photographing lens. Some of such lens barrel driving mechanisms have a helicoid structure. With a mechanism of this type, a movable lens barrel which holds a photographing lens is rotatably received by a stationary lens barrel. The movable lens barrel has a male helicoid formed on its outer peripheral surface while the stationary lens barrel has a female helicoid formed on its inner surface. The female helicoid is designed for engagement with the male helicoid. The movable lens barrel is rotated around the optical axis relative to the stationary lens barrel so that the movable lens barrel may be moved back and forth relative to the stationary lens barrel depending on the direction in which the movable lens barrel is rotated.
In the case of the above-mentioned known helicoid structure, rotation of the movable lens barrel relative to the stationary lens barrel necessarily causes the movable lens barrel to move back and forth relative to the stationary lens barrel so long as the male helicoid is in engagement with the female helicoid. As one counter measure to overcome this inconvenience, it has been proposed to provide an intermediate lens barrel adapted to receive the movable lens barrel, on one hand, and to be received by the stationary lens barrel, on the other hand. In addition, there is provided a mechanism functioning to prevent the movable lens barrel from rotating so that rotation of the intermediate lens barrel causes it to move back and forth relative to the stationary lens barrel and at the same time causes the movable lens barrel to move back and forth relative to the intermediate lens barrel. However, the helicoid structure employing such arrangement has various disadvantages as will be described below.
In order to assure that the movable lens barrel is prevented from rotating, there must be provided a latch member designed for engagement with an appropriate portion of the movable lens barrel, and this engagement must be maintained during linear movement of the movable lens barrel. To achieve this, the helicoid mechanism must be constructed so that the latch member is never disengaged from the movable lens barrel even when the latch member is moving back and forth together with the intermediate lens barrel and operatively associated with the stationary lens barrel so as to be prevented from rotating together with the intermediate lens barrel.
Clearly, operative association between the stationary lens barrel and the intermediate lens barrel, as well as operative association between the intermediate lens barrel and the movable lens barrel, occur through the helicoid structure. This means that the movable lens barrel must be rotated relative to the intermediate lens barrel when the movable lens barrel is telescopically received by the intermediate lens barrel and the intermediate lens barrel must be rotated relative to the stationary lens barrel when the intermediate lens barrel is telescopically received by the stationary lens barrel. In conflict with this requirement, the latch member is to prevent the movable lens barrel and itself from rotating by engaging respective predetermined locations of the stationary, intermediate, and movable lens barrels. Accordingly, the presence of the latch member will necessarily obstruct these lens barrel segments to be smoothly assembled by rotating the movable lens barrel and the intermediate lens barrel. To solve this problem, the latch member is constructed in the form of a separable member or a screw-mountable member so as to be incorporated in the helicoid structure after the respective lens barrel segments have been assembled together. As a result, the number of parts is increased and operation of assembly becomes complicated.